1. Field of the Invention
This invention relates to conductive paint formulations which are useful for coating housings for electronic equipment to provide electromagnetic interference (EMI) and radio frequency interference (RFI) shielding for the housings, and in particular, to paint formulations which may be spray coated onto the housing to provide a thin durable finish which is both adhesive to the housing substrate and cohesive with the conductive pigment and which has low electrical resistivity.
2. Description of Related Art
It is often necessary to provide EMI/RFI shielding for polymeric and other electrically non-conductive enclosures used in electronic and radio equipment to prevent radiation such as EMI/RFI from being emitted from the equipment contained therein. Regulations of the Federal Communication Commission require that certain categories of electronic and computer equipment be enclosed by shields capable of substantially eliminating such EMI/RFI above about 10 kiloHertz (kHz) in frequency.
It is known in general that electrically conducting materials will provide EMI/RFI shielding and, accordingly, it is common practice in the industry to provide a metal coating on enclosures for electronic equipment. Among the techniques known to apply such a coating are the use of electrically conductive paints, flame or arc metal spraying, the application of metal foils and electroless or electrolytic deposition of one or more metal layers.
The many types of electronic equipment which must be shielded from EMI/RFI radiation include commercial, industrial, business and home products such as computers, calculators, video equipment and electronic games. The use of conductive finishes such as electrically conductive paints on the electronic housing however are subject to failure by abrasion, flaking, cracking or corroding allowing leaking of radiation. For this and other reasons, multiple coatings are typically applied for many applications to provide acceptable shielding properties.
To evaluate the electrical effectiveness of a conductive coating on the electronic housing, a test of surface resistivity-measured in ohm/square--termed "ohms per square" is typically used. Both point to point and surface resistivities are usually measured using standard electronic measuring instruments. The durability of the finish and its adhesion to the electronic component substrate is also an important factor in determining the effectiveness of the coating. The conductive paints used to form conductive coatings employ a metallic additive (pigment) in the paint to provide the conductivity of the coating and the ability of the coating to retain the metallic additive (typically termed "metallic pigment" or "pigment") is also a measure of the effectiveness of the coating and is usually termed "cohesion".
Metal containing conductive paints are very popular from an industrial viewpoint to provide a conductive coating on electronic component housings for shielding purposes because of their cost effectiveness. In general, a conductive paint formulation contains a metal such as nickel, copper, and preferably silver and combinations with silver to provide electrical conductivity, a solvent and a binder of generally a polymeric material which are thermoplastic and which on drying form a hard conductive film. The conductive paints are typically applied by spraying with conventional painting equipment and the coated housings are then usually air-dryed. Silver has the highest conductivity and is considered the best type of pigment to use for the most critical applications. The high cost of silver however makes it unattractive for general purpose use. Copper is another metal used in paint but oxidation during use results in loss of conductivity and limits its application. A silver coated copper particle, preferably in the form of flake, has been proposed for use in conductive paints alone as the metal pigment and in admixture with pure silver flake pigment.
At present, conductive paints because of their ease of use and effectiveness are very desirable to use in shielding housings for electronic equipment. There are a number of advantages to the use of conductive paints such as minimal equipment cost, ease of application, and low material cost relative to other conductive finishes. However, coating durability is a significant concern since cracks or chips in the coating or abrasion of the coating will effect the shielding of the coating. The quality of the coating therefore is very important to the shielding effectiveness of the coating. Also, in applying the paint it is usually necessary to employ an agitator or mechanical shaker to keep the relatively heavy metallic particles from settling so a homogeneous mixture is maintained during spray application. In general, for a conductive metal paint, and particularly a conductive paint which is sprayed, it is important that the spray coating be durable, have a high conductivity (low resistivity) and provide a smooth (even) coating. It is also important from a cost and application standpoint that a thin coating be used. Paints even meeting these criteria must also have adhesive and cohesive strength so that the coating adheres to the substrate and the conductive pigment is retained in the paint film. All these properties are now typically balanced because a change in the formulation to achieve a particular result typically has a negative effect on another property. For example, increasing the metallic loading decreases the resistivity but usually causes the paint to have poor cohesion.
In U.S. Pat. No. 5,158,708 an electrically conductive paste is described for screen printing of printed circuit boards to provide an electromagnetic shielding layer. The paste is not used for spray painting of housings for shielding. In U.S. Pat. No. 5,282,887 conductive paints applied by spraying are disclosed containing a pigment grade carbon, a resin, a cross-linking agent and a solvent. U.S. Pat. No. 4,305,847 discloses copper containing compositions for shielding employing organic titanates to stabilize the paint over a long period of time even at elevated temperatures.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an electrically conductive metallic containing paint for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with a low resistivity at thin coatings as low as 0.2 mils and has both excellent adhesion to the substrate and cohesive properties of the coating.
It is another object of the present invention to provide an electrically conductive silver containing paint for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with a low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is a further object of the present invention to provide an electrically conductive silver coated/copper particle containing paint, preferably in admixture with silver particles, for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is a further object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a metal coating, preferably a silver paint coating which is durable, has low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is an additional object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a silver coated copper particle containing conductive paint, preferably in admixture with silver particles, which conductive paint provides a coating which is durable, has low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is another object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a coating which is durable, has low resistivity and provides a smooth finish.
It is a further object of the present invention to provide EMI/RFI shielded housings for electronic equipment which are made using the conductive paints and methods of the present invention.
Other objects and advantages of the present invention will be readily apparent from the following description.